Soil Porosity & Permeability Lab:
Environmental Science AP Instructor: Ben Smith
The nation that destroys its soil destroys itself. –Franklin Delano Roosevelt, in a letter to all State Governors on a uniform soil conservation law, Feb. 26, 1937 (32nd President of the United States)
Soil can be defined as the unconsolidated portion of the earth's crust that is modified through physical, chemical, and biotic. The product of these processes is a medium that is capable of supporting plant growth. The soil volume includes the following components: 1.A mineral portion derived from geologic materials in earth's crust; 2. An organic portion consisting of living, dead, and decomposing organisms and organic residues; and 3. Pore space containing air and water in varying percentages. Pore space is one focal area of this lab activity.
Soil is three-dimensional, with layers (horizons) that vary in arrangement and thickness on different parts of the landscape. The second focal area of this lab involves the arrangement and thickness of particles comprising a given layer of soil, along with the pore spacing of a given soil sample. Soils are not static, but are in a dynamic equilibrium with the surrounding environment.
Objectives: 1. To explore the concepts of porosity and permeability by observing what effect
particle size and shape have on the amount of water that can be held in the open
spaces between particles and also how these two factors impact the rate of water
flow.
A. In the first part of this lab, you will measure and compare the volume of water
needed to fill pore spaces in each sample and then calculate the porosity of
each sample.
B. In the second part of this lab, you will demonstrate the influence of particle size
on permeability. You will measure the amount of water which exited the tube,
then calculate the percentage of water each sample retained, as well as the rate
of drainage.
Part I:Porosity
Materials: Soil Particle Samples: e.g., fine sand, medium sand, fine gravel, medium gravel, and/or a mixture ofsoil particle types, Plastic Beverage Gatorade™ bottles-20 ounce bottles that have the bottom cut off* and caps to serve as “Soil Tubes”, Graduated cylinder, Water, Plastic cups.
*Bottoms may be easily cut off at the indentation located about 2 cm from the bottom of the bottle.
Procedure: 1. Place cap on bottle (soil tube). 2. Fill 100 mL grad. cylinder w/ water. Fill the soil tube w/ water from the grad. cylinder and note the volume of water required to fill inverted Gatorade bottle up to the “neck indention (this is approximately 7 centimeters from the top of the cap when measured as the bottle is upside down). Record this volume in the Data blanks. Empty the tube & dry it before going on. 3. Fill the tube with a soil particle type, such as fine sand (you may repeat steps for coarse sand, gravel, and/or a soil particle mixture); tap togently settle and compact the particles. Add more of particle type to bring particle level up to the bottle indentation located at the 7 cm mark (measured from the cap end.)
4. Fill grad. cylinder w/ water, noting volume. 5. Now slowly pour water into soil tube until the sample is fully saturated and the water level just reaches the top of the soil level.
6. Check the new water level in the grad. cylinder, and record volume of water added to the tube in the Data blanks. 7. Calculate porosity of each of the three samples tested. Porosity is calculated by dividing the volume of the pore space by the total volume of the sample. Enter results for each in Data blanks as a percent. 8. Empty your wet/used soil samples in the marked containers (not the container that you got the dry soil from).
Data:
Volume of water to fill tube(= total V of sample): ______mL.
Volume of water added to fine sand( = total V of pore space): ______mL: Porosity = ____%
Volume of water added to coarse sand(= total V of pore space):______mL: Porosity = ____%
Volume of water added to fine gravel(= total V of pore space): ______mL: Porosity = ____%
Volume of water added to medium gravel(= total V of pore space): _____mL: Porosity = ____%
Porosity = Volume of pore space X 100 = %
Total volume of sample
______
PART II:Permeability
Materials: plastic bottles(with caps) to use as “soil tubes”, screening & rubber bands*, plastic cup, fine sand, coarse sand, fine gravel, medium gravel, graduated cylinder, tap water, funnel, plastic cups. (*note: we may use holey caps in place of banded-screening)
Procedure: 1. Fasten screening over one end of soil tube with rubber band. 2. Fill tube with a given soil sample or soil particle type as you did in the porosity test(s) (you may repeat w/ other soil particle types or a mixture).
3. Tap gently to settle/compact sample. 4. Fill grad. cylinder w/ water to 50 mL mark.
5. While holding soil tube over a plastic cup, pour 50 mL of water into soil tube. Time &
record how long it takes for all water to drain through tube. 6. When water has drained, measure amount in grad. cylinder andrecord value. 7. Calculate percentage water retained by the soil particle type by subtracting amount of water drained into the cup from the 50mL amount originally added. 8. Divide this difference by the original 50mL value to find the decimal amount and then multiply by 100 to arrive at the percentage. 9. Now calculate the rate of drainage for each soil particle type by dividing the amount of water drained into the cup by the amount of time it took the water to drain.
10. Return “used” soil samples to marked containers. -Thank you.
Data:
Sample Drainage Time
Fine Sand ______seconds/minutes
Coarse Sand ______seconds/minutes
Gravel(fine) ______seconds/minutes
Gravel (medium) ______seconds/minutes
Volume of water in cup (Fine Sand) ______mL
Volume of water in cup (Coarse Sand) ______mL
Volume of water in cup (Fine Gravel) ______mL
Volume of water in cup (Medium Gravel) ______mL
Retention % = Volume of water added(50mL) – Volume of water in cup X 100
Total volume of water added (50 mL)
Retention Percentage (Fine Sand) ______%
Retention Percentage (Coarse Sand) ______%
Retention Percentage (Gravel) ______%
Drainage Rate for Fine Sand ______mL/sec. (cubic centimeters/sec.)
Drainage Rate for Coarse Sand ______mL/sec.
Drainage Rate for Fine Gravel ______mL/sec.
Drainage Rate for Medium Gravel ______mL/sec.
Lab-related Questions: You will (probably) need to spend a few minutes researching a few of the questions below. The Botkin & Keller text does not address all of the items below.
- Of the four soil particle types (clay, silt, sand, and gravel) which oneis most likely to become waterlogged (support a standing body of water)?
- Draw (with your hand) the classic Soil Texture Triangle, providing labels and other essential aspects of this time-honored and widely used soil diagram. (Note: 12 different soil types/mixtures are typically presented in the Soil Texture Triangle diagram.)
- Which type of soil, in terms of soil particle type and percent composition, would likely be conducive to growing agricultural crops, in general? Describe this soil type by presenting major characteristics that this soil (type) should possess.
- Defineloam (based on the percentage of non-gravel soil particle types in “loam”):
- Besides topography, describe four other major factors which determine the properties of soils:
- Besides in a ceramics application or use and besides the athletic field application, describe one other widespread use of clay that relates to/is part of a comprehensive environmental science course. (If drawing a blank, you may be moved to flip through Botkin & Keller to find a major clay use to fit the bill here.)
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